1. Field of the Invention
The present invention concerns a process for producing cables, in particular cables for the distribution of electrical energy or cables for telecommunications.
More particularly, the present invention concerns a process for the production of cables having at least one covering layer comprising a polymeric base composition having high viscosity.
Still more particularly, the present invention concerns a process for the production of cables having at least one covering layer comprising a polymeric base composition to which is added a mineral filler capable of imparting one or more specific properties to the aforesaid cables.
The present invention further concerns a device for the purpose of performing the production process referred to above.
2. Description of the Related Art
In general, an electric cable comprises at least one conducting element, consisting of a single wire or of a plurality of wires stranded together in a suitable way, and one or more covering layers of the aforesaid conducting element, which provide electrical insulation and/or fulfil a function of mechanically or chemically/physically protecting the cable against external factors.
This covering layer or several covering layers are applied onto the conducting element via a deposition stage, generally performed by means of an extruder.
In general, an extruder comprises: a hollow cylindrical casing; an extrusion screw of preset pitch positioned within said casing and having an axis of rotation parallel to the axis of said cylinder; a charging hopper located at a first end of said casing for the introduction of a predefined composition based on at least one polymeric material, optionally premixed with other components in an upstream device, such as for example a Banbury mixer; a filtration section located close to the head of said screw, positioned perpendicularly to the axis of the latter and thus perpendicularly to the direction of advancement of said composition; a connecting flange positioned downstream from the filtration section; an extrusion head comprising a conveyor element and a die communicating with the exterior, so as to define the second end of said casing, and whose purpose is to impart a predefined shape to the material emerging from the extruder.
More specifically, according to a form of embodiment known in the art, the extrusion head is provided with an inlet port through which the conducting element to be covered with the aforesaid covering layer is introduced inside of the extruder.
According to a technology known in the art, the conducting element is introduced into the extruder head perpendicularly to the direction of advancement of the material fed into the extruder through the hopper referred to above.
In the case where the cable production process includes the use of an extrusion operation, said process then comprises the following stages:
feeding the extruder with the composition forming the covering layer which it is desired to deposit on the conducting element;
unwinding a conducting element from a feeding reel and conveying it inside of the extruder head where the deposition of said covering layer onto said conducting element is effected;
cooling the cable thus obtained and winding it onto a collecting reel.
Said polymeric material and said other components can be premixed together in a device upstream from the extruder, creating a mixture which is fed into the latter via the charging hopper referred to above.
Furthermore, prior to said cooling operation, a crosslinking operation can be performed in the case where polymers of crosslinkable type are used.
This type of process known in the art, includes at least the following devices for the purposes of its implementation:
at least one charging hopper for feeding in the polymeric material, optionally premixed with other components of said composition;
at least one extruder comprising an extrusion screw and an extrusion head inside of which is contained a die for the purpose of fitting said covering layer around at least one conducting element of said cable;
one or more units for cooling the cable thus produced;
devices for unwinding the conducting element, and
devices for winding the cable downstream from the production plant.
As stated above, said device can also include one or more crosslinking units if a polymeric material of crosslinkable type is used.
It is also known that the extrusion operation can take place in several distinct stages, above all in the case where it is desired to cover the conducting element with a plurality of covering layers.
For example, if it is desired to deposit a pair of covering layers onto the conducting element, the extrusion operation can involve, in a first step, the extrusion of an internal covering layer, in direct contact with the conducting element, and then, in a second step, the extrusion of an external covering layer, deposited on said internal covering layer.
This covering process can also take place in a single step, for example by a xe2x80x9ctandemxe2x80x9d technique, in which at least two individual extruders placed in series are used, or by coextrusion with a single extrusion head.
In the present description and in the claims which follow, the term xe2x80x9ccovering layer of a cablexe2x80x9d means any covering comprising at least one polymeric material deposited on the conducting element of said cable, where this covering may consist of one or more layers, each having, for example, electrical insulating properties or being capable of protecting the cable from the action of external factors.
Furthermore, for simplicity of explanation, in the course of the present description the expression xe2x80x9coperation of extrusion of a covering layer onto the conducting element of a cablexe2x80x9d should be taken to mean an extrusion operation performed onto the conducting element itself, in the case where it is intended to make a cable provided with a single covering layer, or an extrusion operation performed onto a covering layer previously deposited onto the conducting element, in the case where it is intended to make a cable provided with a plurality of covering layers.
In addition to the stages mentioned above, in general the production processes of a cable, upstream from the deposition stage of the covering layer onto the conducting layer, include the provision of a filtration stage for the purpose of removing the impurities present within the components that form the composition referred to above.
This is because these impurities can be contained inside of said components, for example if the latter are fed into the extruder in the form of granules or pellets, in which case the impurities are embedded in the interior of the granules or pellets; or said impurities are introduced together with said components as a result of the interaction of the latter with the external environment during the usual operations of handling, transport or storage to which said components are subjected.
Furthermore, not only the quantity of impurities present in the covering layer, but also the dimensions of said impurities, are of fundamental importance.
This aspect is particularly critical, for example, in the case where the covering layer of a cable for use at high tension, for example 150 kV, has to be produced, a case where it is necessary to confine the dimensions of the impurities to below a critical value, for example less than 300 xcexcm, more preferably less than 150 xcexcm.
In particular types of application, for example when it is desired to produce a cable of the self-extinguishing type, the external covering layer of the conducting element is provided with a high percentage of mineral filler, which imparts the desired flame-resistant properties.
The presence of the mineral filler renders the material very viscous and considerably worsens its processability. In particular, the filtration stage of the process of extrusion of a material containing flame-resistant mineral fillers is particularly critical, even though for this type of cable impurities of relative large dimensions are tolerated and thus a less stringent filtration operation than that required for high tension cables is necessary.
The filtration stage included in an extrusion process, besides removing the impurities referred to above, has the purpose of removing any lumps of material formed as a result of imperfect mixing and plastification during the extrusion of the components of the starting composition.
If there were no filtration operation present downstream from the extruder, said lumps would result in the occurrence of defects in the covering layer of the finished cable, adversely affecting the properties of the latter.
However, it is known that the introduction of a filtration stage into a production process of the continuous type, such as for example the process of production of a cable, involves the occurrence of pressure losses within the production unit due to the passage of the material to be filtered through that filtration section.
The patent U.S. Pat. No. 5,182,066 aims to solve the problem of the pressure losses by inserting a pump within the production process, downstream from the filtration section.
More specifically, U.S. Pat. No. 5,182,066 describes a cable production process comprising an extruder, a hopper for feeding into the extruder the components of the covering layer which it is desired to be deposited onto the conducting element of said cable, a filtration element for the removal of the impurities present within said covering layer, a pump for the purpose of providing a suitable pressure to the material emerging from the filtration section, a mixer to mix the filtered material with a crosslinking agent, a further extruder into which is fed the conducting element onto which, finally, the covering layer is deposited.
The patent U.S. Pat. No. 5,182,066 referred to above aims to reduce the quantity and dimensions of the impurities contained in the covering layer, applied to an electrical cable by means of an extrusion operation, maintaining within acceptable limits the operating pressures at which operate the devices, including the extruder, constituting the production unit.
The solution proposed in that document thus involves the use of a device, such as for example a pump, which, positioned downstream from the filtration section, is capable of imparting energy to the filtered material and pushing it towards the extrusion head.
The provision of said device would thus make it possible to solve the problem of the pressure losses due to the interposition of a filtration section.
This solution would also be applicable if it were desired to use an extruder designed to operate at limited pressure values.
However, the solution proposed in the patent U.S. Pat. No. 5,182,066 inevitably involves a major complication both as regards the design of the extrusion plant, the latter necessitating a further device for moving the material, and as regards the costs of the production process as a whole.
The Applicant has found that a process for producing cables, in particular cables for the distribution of electrical energy or cables for telecommunications, can be carried out under less severe operating conditions than those of the production processes of the known art by effecting an increase in the filtration efficiency, for an equal filtration area used.
The Applicant has perceived that the pressure losses present in a filtration section are due to the sum of two distinct contributions: the pressure losses due to the presence of the filter screens and the pressure losses generated by the presence of a filter support plate capable of supporting said filter screens.
Thus the Applicant has found that, for an equal filtration area used, this increase in the filtration efficiency can be obtained by using a filter support plate of the sectored type described in detail later in the course of the present description.
The Applicant has further found that this increase in the filtration efficiency is particularly advantageous in processes for the production of cables whose covering layer comprises, together with the polymeric base composition, a suitable mineral filler capable of imparting one or more specific properties to the aforesaid cables.
In the rest of the present description and in the claims which follow, xe2x80x9cuseful filtration areaxe2x80x9d (Au) means the filtration area rendered effectively available for the passage of the material to be filtered, after excluding the obstruction area of the meshes of each filter screen, where said area can in fact vary from case to case depending on the number of filter screens used and on the diameter of the wires which form each mesh. More particularly, with the following definitions:
a) A0: the total area of the cross section of the duct where the filtration section is positioned, and
b) As: the area of the cross section of the duct occupied by the filter support plate which supports the filter screens in the correct operating position,
the useful filtration area is given by: Au=A0xe2x88x92As.
Furthermore, let us define xe2x80x9cfiltration efficiencyxe2x80x9d (E) as the ratio between the aforesaid useful filtration area and the total area of the cross section of the duct.
Thus:E=(Au/A0)=(A0xe2x88x92As)/A0
where the filtration efficiency, as stated above, does not take account of the obstruction due to the filter screens. In other words, for the same geometry and location of the filter screens, the filtration efficiency depends on the obstruction due to the filter support plate.
In accordance with the above, in a first aspect, the invention concerns a process for the production of a cable having at least one covering layer consisting of a composition based on at least one polymeric material by the use of an extruder, said extruder comprising a cylindrical casing, at least one extrusion screw of preset pitch positioned within said casing and having an axis of rotation parallel to the axis of said cylinder, a charging hopper located at a first end of said casing, a filtration section located close to the head of said screw, and positioned perpendicularly to the axis of said screw, a connecting flange positioned downstream from the filtration section, and an extrusion head comprising a conveyor element and a die communicating with the exterior, so as to define a second end of said casing, said process comprising the stages of:
conveying at least one conducting element into the interior of said extruder;
feeding the polymeric material, optionally premixed with other components of said composition, into said extruder via said charging hopper;
filtering said composition transferred and plasticized by said extrusion screw;
depositing said composition onto said at least one conducting element, characterized in that the filtration operation is performed with a filtration efficiency greater than 0.8.
In the process according to the invention, the aforesaid filtration operation is performed using a filter support plate of the sectored type, generally positioned downstream from said extrusion screw.
In a second aspect, the present invention concerns an extruder for the production of a cable having at least one covering layer consisting of a composition based on at least one polymeric material, said extruder comprising: a cylindrical casing; at least one extrusion screw of preset pitch positioned within said casing and having an axis of rotation parallel to the axis of said cylinder; at least one charging hopper located at a first end of said casing; a filtration section located close to the head of said screw, and positioned perpendicularly to the axis of said screw, said filtration section comprising at least one filtration element held by a support element; a connecting flange positioned downstream from said filtration section, and an extrusion head comprising a conveyor element and a die communicating with the exterior, so as to define a second end of said casing, characterized in that said support element is a plate of the sectored type.
According to the present invention, said plate of the sectored type comprises a structure of truncated conical shape provided with a plurality of support elements for said at least one filtration element, said support elements being located bearing onto the aforesaid truncated conical shape and extending radially towards the interior of said structure.
Further characteristics and advantages will appear more clearly in the light of the following description of a preferred embodiment of the present invention.